When a chimney leaks, the water rarely enters where you see the stain. More often it slips in at the roofline, through a thin band of metal most homeowners never think about: the flashing. In a community like Lemont, where older homes and hard-driving storms are both common, failed flashing is one of the most frequent reasons we get called out for a leak. Understanding how it works makes the difference between a lasting repair and an endless cycle of caulk.
What flashing does and why it matters
Your chimney punches straight through the roof, creating a seam that rain would love to exploit. Flashing seals that seam. Done correctly it is not one piece of metal but a layered system: step or base flashing woven under the shingles, and counter flashing set into a groove cut in the masonry and folded down over the base. Water hitting the joint is guided out onto the shingles rather than down into the house. When that layering is intact, the chimney stays dry. When it fails, water has a direct path into the roof structure.
Why flashing leaks — and how to spot it
Flashing does not last forever. The metal expands and contracts through every hot day and cold night, fasteners work loose, and any sealant used at the joint dries out and cracks. The mortar holding the counter flashing erodes over the years, opening the very gap the flashing was meant to close. Poor original installation is a major factor too — a single bent sheet of metal, or a reliance on roofing tar instead of proper counter flashing, is destined to leak.
The signs show up indoors first: water stains on the ceiling or wall around the chimney, damp or discolored framing in the attic near the penetration, a musty smell, or rust streaks running down the brick. Because the water travels before it appears, the stain can be a foot or more from the actual entry point.
Patching versus a real repair
It is tempting to smear on more sealant and call it done, but caulk and tar are temporary. They harden, crack, and leak again within a season or two, and each new layer just buries the failing joint. A durable repair re-seats or replaces the flashing and cuts fresh counter flashing into the masonry so the metal, not the sealant, does the work. Because flashing leaks often travel with crown, cap, or masonry problems, we look at the whole system during a chimney repair visit rather than chasing one stain at a time.
If you have noticed a stain near your chimney or watched a patch fail again, do not wait for the next storm to make it worse. Call Indigo Chimney Sweep at (815) 296-9565 or book online. We are bonded and insured, and we tell you the price up front. We will find where the water is really getting in around your Lemont chimney and fix it so it stays dry.
Frequently asked questions
What is chimney flashing?
Flashing is the layer of sheet metal that seals the joint where the chimney passes through the roof. It is built from a base flashing lapped under the shingles and a counter flashing set into the masonry, overlapping so water is guided away from the gap instead of running into it.
How do I know the leak is coming from the flashing?
Flashing leaks usually show up as water stains on the ceiling or wall right around the chimney chase, damp framing in the attic near the penetration, or rust streaks on the chimney. The staining appears during or just after rain and often worsens with wind-driven storms.
Why does flashing fail?
Metal expands and contracts with temperature, sealant dries and cracks, fasteners loosen, and the mortar joint holding the counter flashing erodes. Improper installation is common too, especially a single bent piece of metal instead of properly layered step and counter flashing.
Can I just add more roofing tar or caulk?
Sealant is a short-term patch, not a repair. It buys time but hardens and cracks within a season or two, and piling on more only hides the failing joint. A lasting fix means re-seating or replacing the flashing and cutting fresh counter flashing into the masonry.
Is a chimney leak really urgent?
Yes. Water that gets past the flashing rots roof decking and framing, stains ceilings, feeds mold, and works into the chimney where it accelerates masonry and liner damage. A small flashing repair now is far cheaper than the structural and interior damage a slow leak causes.
Does flashing have anything to do with the crown or cap?
They are separate defenses against the same enemy: water. Flashing seals the roofline, while the crown and cap keep rain out of the flue at the top. A thorough leak diagnosis checks all three, because chimney leaks often have more than one entry point.
How long should new flashing last?
Properly installed, correctly layered flashing in sound metal can last for decades, often as long as a quality roof. What shortens its life is thin material, reliance on sealant alone, and skipped maintenance. Having it checked during your annual chimney service catches wear before it leaks.
Should I fix flashing before selling my home?
It helps. Water stains around a chimney are a red flag for inspectors and buyers, and an active leak can stall a sale. Resolving it and keeping the paperwork shows the chimney and roof were maintained, which is reassuring at the closing table.
Written by Jamie Reed, operations manager at Indigo Chimney Sweep.
Published September 12, 2026